Such miniature metering pumps already exist that are suitable for fitting to a receptacle and that comprise a body provided with an inlet associated with an inlet valve, and a plunger piston that is urged towards a rest position by a resilient return member, while also having a hollow rod or axial nozzle that projects from said body to serve both as a push rod and as a tube for ejecting substance, its internal channel being associated with an outlet valve. As a general rule, the inlet valve member is unbiased, however the valve member of the outlet valve is rated, i.e. biased towards a closed position by a resilient member. By virtue of a sealing gasket, the body and the piston co-operate to define a metering chamber which initially contains a certain volume of air that is generally exhausted, when priming the pump, by the outlet valve member being opened mechanically.
The resilient members are often in the form of metal springs and they are not always suitable for coming into contact with the substance, nor are they suitable for being subjected to the same reprocessing operations as the other parts of the pump if the pump is recycled.